The invention relates to modified polyisocyanates and polyisocyanate mixtures, a process for their preparation and their use as starting components in the preparation of polyurethane plastics, in particular as crosslinking agents for water-soluble or xe2x80x94dispersible paint binders or binder components with groups which are reactive towards isocyanate groups.
Against the background of increasingly stricter environmental legislation, water-dispersible polyisocyanates have gained importance for various fields of use in recent years. They are currently used in particular as crosslinking components for high-quality water-dilutable two-component polyurethane paints (2C PU paints) or as an additive for aqueous dispersion adhesives, are used for crosslinking aqueous dispersion in textile finishing or formaldehyde-free textile printing inks and moreover are also suitable, for example, as auxiliary substances for wet-strength finishing of paper (cf. e.g. EP-A 0 959 087 and literature cited herein).
In practice, practically exclusively nonionic polyisocyanates modified hydrophilically with the aid of polyethers are currently employed for all these fields of use. The preparation of such water-dispersible polyisocyanates is discussed in detail, for example, in EP-A 0 959 087, page 2, lines 25-46.
In spite of their wide market acceptance for the most diverse uses, however, polyether-modified polyisocyanates have a number of main disadvantages. Because of a very high viscosity maximum which is to be overcome during dispersing, for example, they can often be incorporated homogeneously into aqueous media only by applying considerable shear forces (e.g. high-speed stirrers). The high polyether content required for adequate dispersibility, in particular for use as crosslinking agents in aqueous 2C PU paints, furthermore imparts a permanent hydrophilicity to the coatings obtained.
To bypass these disadvantages, attempts have also already been made to prepare self-dispersible polyisocyanates modified hydrophilically by incorporation of ionic groups.
EP-A 0 443 138, EP-A 0 510 438 and EP-A 0 548 669 describe, for example, polyisocyanate mixtures which contain chemically bonded carboxyl groups. Such polyisocyanates can indeed be stirred into aqueous systems in very fine division after neutralization of the carboxyl groups without high shear forces being necessary, but they have a completely inadequate storage stability, especially in the neutralized form. Because of the known catalytic activity of the carboxylate group, a polymerization of the isocyanate groups already starts at room temperature, for example with trimerization to polyisocyanurates or formation of xcex1-nylon structures, which as a rule leads to gelling of the product after a few days.
EP-A 0 703 255 describes ionically hydrophilized water-emulsifiable polyisocyanates which comprise, as emulsifiers, reaction products of polyisocyanate and any desired hydroxy-, mercapto- or amino-functional compounds with at least one sulfuric acid group or anion thereof. Sulfuric acid builder components which are mentioned here as preferred for the preparation of the emulsifiers are hydroxysulfonic acids with aliphatically bonded OH groups or salts of such hydroxy-sulfonic acids, for example specific polyether-sulfonates, such as are marketed e.g. under the name Tegomer(copyright) (Th. Goldschmidt A G, Essen, D E), bisulfite adducts on unsaturated alcohols, such as are obtainable e.g. in accordance with the doctrine of DE-A 2 417 664, DE-A 2 437 218 or DE-A 2 446 440, hydroxyethane- and hydroxypropanesulfonic acid and aminosulfobetaines, which can be prepared by quaternization of tertiary amino alcohols with 1,3-propanesultone. However, these hydrophilizing agents also have a number of disadvantages.
Thus, for example, hydroxypropanesulfonic acid is in equilibrium with its anhydride, 1,3-propanesultone, which is classified as carcinogenic. It can therefore be handled on an industrial scale, in particular, exclusively in the form of aqueous solutions and is consequently unsuitable in principle as a builder component for the preparation of modified polyisocyanates.
On the other hand, hydroxyethanesulfonic acid, polyether-sulfonates of the Tegomer(copyright) type and the bisulfite adducts on unsaturated alcohols mentioned are also available as anhydrous products in the form of their sodium salts on a large industrial scale. The use of these sodium salts indeed in principle allows the preparation of water-emulsifiable polyisocyanates, but these have only a very limited suitability for use as crosslinking components in aqueous paint systems. Because of the only low compatibility of alkali-neutralized sulfonate groups with conventional paint binders, their use in aqueous 2C PU paints in general leads to cloudy, in some cases inhomogeneous coatings. In contrast to the volatile neutralization amines conventionally employed in dispersions, the sodium ion remains in the paint film even after curing and imparts a permanent hydrophilicity to this.
All the hydroxysulfonic acids proposed as hydrophilic components in EP-A 0 703 255 moreover lead, as the concrete embodiment examples of this publication demonstrate, as a rule to significantly yellow-coloured polyisocyanates, which also obstructs a use of these products as crosslinking components in high-quality paint systems. For the reasons mentioned, polyisocyanates modified with sulfonate groups have not yet been able to establish themselves on the market.
The object of the present invention was therefore to provide new water-dispersible polyisocyanates which are suitable for all the fields of use of water-dispersible polyisocyanates and do not have the disadvantages of the prior art. These new polyisocyanates should be based on readily accessible, toxicologically acceptable builder components which allow a free choice of the neutralizing agent, and in particular should be readily compatible with conventional paint binders.
It has been possible to achieve this object by providing the water-dispersible polyisocyanates or polyisocyanate mixtures according to the invention which are described below in more detail. To simplify the description of the present invention, in the following the term xe2x80x9cpolyisocyanatesxe2x80x9d synonymously also means mixtures of various polyisocyanates.
The present invention is based on the surprising observation that, in spite of their melting points of above 300xc2x0 C., 2-(cyclohexylamino)-ethanesulfonic acid and 3-(cyclohexylamino)-propanesulfonic acid, which in general are used as zwitter-ionic biological buffer substances, can already be reacted with polyisocyanates under very mild reaction conditions in the presence of a suitable neutralization amine, storage-stable, light-coloured products which can be emulsified in water in finely divided form being obtained. This was surprising, since a number of other aminosulfonic acids which are very similar in structure cannot be reacted with polyisocyanates even under considerably more drastic conditions.
Although the use of compounds containing sulfonate groups for the preparation of hydrophilic polyisocyanates is co-mentioned globally in some publications, for example EP-A 0 061 628 and EP-A 0 206 059, the subject matter of which is polyether-modified polyisocyanates, and hydroxysulfonic acids and aminosulfonic acids are also mentioned as suitable builder components for water-dispersible crosslinking agents in EP-A 0 469 389, it has not been possible for the expert to obtain any indication from these publications, as in the same way from the doctrine of EP-A 0 703 255, of the particular suitability of 2-(cyclohexylamino)-ethanesulfonic acid and 3-(cyclohexylamino)-propanesulfonic acid for the preparation of water-dispersible polyisocyanates.
The present invention therefore provides modified polyisocyanates which are obtainable by reaction of polyisocyanates with 2-(cyclohexylamino)-ethanesulfonic acid and/or 3-(cyclohexylamino)-propanesulfonic acid. These are dispersible in water after neutralization of at least a proportion of the sulfonic acid groups. The invention also provides the use of these sulfonic acids for the preparation of water-dispersible polyisocyanates.